The problem of continuous measurement of the mass of consumable electrodes in electric furnaces, especially vacuum-arc ones, is still far from being solved. Determining the electrode mass in the course of remelting is necessary to ascertain the exact instant the forming shrink hole regulation must be started, the end of the melting process and instantaneous values of the consumable electrode melting rate. Complicated remelting conditions, namely, the passage of melting current, often flowing through measuring instruments, electric and magnetic fields, arc discharge interferences, thermal radiation of a molten metal bath, pressure in the furnace chamber, dynamic loads arising in electrode replacement and arc striking, all impede the measurement of the electrode mass and introduce errors commensurate with the measured value.
As for mass transmitters employed in devices for measuring the mass of consumable electrodes, use is made of strain gauges (see, e.g. GFR Pat. No. 1157739, 1964 or US Pat. No. 3179734, Cl. 13-12, 1965) set up in the rope suspension of a rod on which the consumable electrode is fixed. In the above strain gauge arrangement, a main measuring error stems from friction in rod packing, from change in weight of electrical cables and water hoses, as well as from an increase in coolant pressure as the electrode is melted off and the rod descends.
Also known are devices (see, e.g., Alperovich M. E., Beljanchikov L. N., Laktionov V. C. "Programme control of vacuum-arc remelting using consumable electrode" magazine "Metallurgist", No. 4, 1971, pp. 14 - 16), comprising apart from a strain gauge a counter for measuring the electrode mass in terms of its melted-off portion (its length). The counter is associated through a selsyn drive with an electrode holder. However, the above devices fail to provide adequate measuring accuracy due to variation of the density and cross-section of the electrode along its length. Low accuracy of measurement precludes the use of the devices for monitoring the remelting process, especially at its final stage, for estimating the ionization and side discharge effects on the electrode remelting rate.
The readings of the sensitive elements of mass transmitters are also affected by the loop configuration of the cables connected to the rod, which configuration is dependent on the magnitude of the current flowing therethrough.
An American firm "Consarc" has designed a ring-type transmitter (see "Electroslag remelting", a translation from English under the editorship of Corresponding Member of the Academy of Sciences of the Ukrainian Republic Medovar, Kiev, 1973, p. 68) which is mounted directly under the vacuum sealing of the rod. With such arrangement the influence of cable weight on the accuracy of measuring the electrode mass is completely eliminated, though it is partially affected by the weight of the rod proper introduced into the furance chamber and by the pressure of the liquid cooling the rod.
To enhance both the sensitivity and accuracy of measurement of the mass of a consumable electrode, provision can be made for various types of mass transmitters arranged between the rod and the electrode holder. These include Hereus dynamometers (GFR), force transmitters (see, e.g., US Pat. No. 3379818, 1968), magnetostriction transmitters (see, e.g., U.S. Pat. No. 3272905, Cl. 13-9, 1966).
However, the accuracy of measuring the electrode mass by the devices provided with the above transmitters is affected by all the above-specified interferences whereby they do not ensure, in the long run, an adequate measuring accuracy in spite of a high sensitivity of the transmitters themselves.
Also known are devices for measuring the mass of an electrode being remelted in electric furnaces, comprising a hollow cooled rod with a mass transmitter casing secured thereto, the mass transmitter movable element being constituted as a bar with an annular ring rigidly coupled with a consumable electrode holder, the working surface of the annular projection interacting with a sensitive element of said transmitter (see, e.g., US Pat. No. 3272905, Cl. 13-9, 1966).
In the above device, some of the current flowing from the rod to the electrode passes through the magnetostriction-type sensitive element constituted as a winding with a core, the magnetic permeability thereof being a function of the load applied to the core. The magnetic field set up by the current flowing through the sensitive element can also saturate the core, and thus change its magnetic permeability. Different melting currents may cause scatter in the readings of magnetostriction sentitive elements. Moreover, the magnetic permeability of the above cores and the loading force are in a non-linear relationship.
In the device of the type described, only that portion of the rod which is arranged above the magnetostriction elements is exposed to the effect of the coolant. In the above design, the sensitive elements are inevitably heated by the electrode holder. Since it is commonly known that magnetic permeability is temperature-dependent, the measuring error introduced by a change in magnetic permeability becomes inevitable in this case. Errors in measuring with the above device can be the result of the friction of the movable element caused by its deformation under the effect of the electrode mass. Moreover, a small value of the output signal of the mass transmitter hampers the measurements of these signals, differing in microvolts, during long remelting processes lasting many hours and calls for the use of highly sensitive precision instruments.